This invention relates to a method and apparatus of continuous separation of the components of a mixture, and, more particularly, to a compact, highly efficient apparatus and method operable to separate the components of a binary mixture without the aid of gravity.
Distillation is the most commonly used separation technique in petroleum refineries, chemical plants and other process industries. The main purpose of distillation is the separation of volatile components from non-volatile components, or the separation of the mixture of volatile components. The most common distillation technique is fractional distillation in which one or more components of a mixture in vapor form are separated from other components in liquid form in a countercurrent or stepwise countercurrent operation.
Referring to FIG. 1, a simplified example of a continuous fractional distillation system is illustrated. The system comprises a fractionating column filled with packing material such as glass, ceramics, polymers, metal or other materials which are not affected by the operating conditions within the column. Alternately, the distillation column may be provided with a plurality of spaced trays formed in any one of a number of known configurations. A still or reboiler is connected at the bottom of the fractionating column and is provided with a suitable source of heat such as steam. An outlet line is connected to the top of the fractionating column leading to a condenser.
In a simple continuous fractional distillation operation, a feed mixture to be separated is continuously introduced approximately midway along the fractionating column, and flows downwardly by gravity toward the bottom. A portion of the feed mixture from the column is vaporized in the still and the vapor rises vertically upwardly from the bottom of the column making contact with the descending liquid. When it reaches the top of the column, the vapor exits through the outlet line and is condensed in the condenser. A portion of that condensate is removed as product and the remainder is returned to the top of the column in liquid form, known as reflux, which then flows downwardly by gravity within the column.
The flow rates of liquid and vapor are adjusted within the column so that near the upper end of the column the liquid has a higher concentration of more volatile components than corresponds to equilibrium with the vapor with which it is in contact. Therefore, the more volatile components of the feed material pass from the liquid to vapor stage, and the less volatile components pass from the vapor to liquid stage producing a countercurrent flow of more volatile components toward the top of the column and less volatile components toward the bottom of the column. The vapor becomes progressively more enriched in volatile components as it flows to the top of the column, and the liquid becomes more concentrated in less volatile components as it flows to the still where it is removed as a bottoms product.
The separation of the components of a feed mixture in a continuous fractional distillation system depends on the relative volatility of the feed components, the ratio of the liquid to vapor in the column or the fraction of condensate returned as reflux compared to that removed as product, and the effectiveness of the transfer between the liquid and vapor phase provided by the column. Relative volatility of the feed components is not subject to change, and the fraction of condensate or reflux returned to the column as compared to the product removed is essentially an operating parameter. Therefore, improvement in the efficiency of the column in effecting a transfer between the liquid and vapor phases of the feed mixture is the aspect of distillation system operation which has received the most attention in recent years. A common goal of all fractional distillation columns, including both tray tower and packed tower types, is to provide a large surface area of contact between the liquid and the vapor phase. Numerous plate designs have been proposed to achieve more complete contact between the descending liquid and the ascending vapor. Packed columns, using a variety of materials, have been designed to create a similar agitated countercurrent flow of liquid and vapor.
Despite recent improvements in fractional distillation column design, problems including flooding, liquid dispersion and interstage cooling remain. In addition, separation efficiency is limited by the physical height of the column. It has been found that the separation efficiency of both tray and packed columns, particularly for feed mixtures having components with relatively close boiling points, increases with the height of the column wherein additional surface area of contact between the liquid and vapor phases is provided. However, increasing the height of fractionating columns or towers adds significantly to the cost of the distillation system and may present a problem in installations having limited available space. A further disadvantage of all known continuous fractionating columns is that they depend on the operation of gravity to produce a countercurrent flow of descending liquid and ascending vapor, and therefore must be oriented vertically to operate.
As is well known, heat pipes are commonly used in heat exchangers and other heat transfer devices to transmit heat from one end of the heat pipe to the other. In the vast majority of applications, a pure liquid is used to convey the heat within the heat pipe. A countercurrent flow of the pure liquid in vapor and liquid phase is established in the heat pipe, and since the liquid and vapor are in continuous contact, there is a continuous exchange of mass between the two phases. It has been found that if a binary mixture is placed in the heat pipe, the concentration of the components forming the mixture will be polarized such that the more volatile component is concentrated at the low temperature end of the heat pipe and the less volatile component of the mixture is concentrated at the higher temperature end. If the heat pipe does not contain inlets or outlets, a total reflux situation is produced at steady state.
The operation of a two-component heat pipe using a binary mixture resembles that of a fractionating column at total reflux. However, a fundamental difference exists between the two systems resulting from the different roles played by their operating pressures. The system pressure in a distillation column is preset, typically at atmospheric pressure. The pressure inherently developed inside the closed system of a heat pipe, however, is influenced by a variety of system parameters, such as the type of mixture placed in the heat pipe. Unlike fractionating columns, heat pipes are closed systems which inherently develop their own pressure differential to drive the vapor phase of the mixture in one direction, and the liquid phase in the opposite direction along a wick structure, independently of gravity or other external forces. While separation of the components of a binary mixture is achieved in a two- component heat pipe, known heat pipes of that type have been used exclusively to transmit heat from one end to the other.
Another type of separation apparatus used in the process industry is the sorbent column. Typical sorbent columns include a sorbent material such as silica gel or activated charcoal in which each component of a mixture moves in the same direction in the adsorbed phase. While sorbent columns provide some advantages over distillation columns, their separation efficiency is dependent on the permeation rates of the components to be separated. In addition, the operation of known sorbent columns is not continuous but limited to batch operations or an alternating operation with both adsorption and desorption cycles.